Zoomimng beyond basemap max scale

In my program I have a map that consists of 2 layers. 1st one is a basemap World_Light_Gray_Base (rest), the 2nd one is a LocalTiledLayer and I load it from androids external memory (it's a .tpk). The problem is that layers of the .tpk file appear only once you zoom beyond 1:150 (it's intentional, I use it for building interior) but there seems no way to zoom beyond that point, the maximum scale I am able to get is like 1:450 or something like that (it's the maximum of basemap, I assume). I tried using zoomToScale, but it still wont go beyond 1:450 (or whatever the maximum of that basemap is)...